Method and apparatus for controlling the load placed on a compressor

ABSTRACT

A method of operating a vapor compression system, the vapor compression system defining a closed fluid circuit in which a refrigerant is circulated and having operably disposed therein, in serial order, a compressor, a high pressure heat exchanger, an expansion device and a low pressure heat exchanger. The method includes applying a variable thermal load on a first one of the heat exchangers, monitoring the thermal load placed on the first heat exchanger and controlling the operation of the system to limit the thermal load placed on the first heat exchanger when the thermal load exceeds a predetermined value. A heat exchange subsystem employed to limit the thermal load may include reducing the flow of a heat exchange medium over the heat exchanger or to recirculate the heat exchange medium in a manner which reduces the thermal load on the heat exchanger.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for controllingthe load placed on a compressor and, more particularly, the load placedon a compressor used with a refrigerated cabinet.

Refrigerated cabinets and the refrigeration systems which cool suchrefrigerated cabinets experience variable load conditions. The variableload conditions may occur due to a temperature increase of the contentsof the refrigerator, such as when warm objects are placed in therefrigerator. Further, changes in the temperature of the ambientenvironment or frequency and duration at which users access therefrigerated cabinet will also vary the cooling load placed on therefrigeration system.

Refrigerated cabinets, as may be found in grocery stores or used asvending machines for cooled products, may employ a CompleteRefrigeration System (“CRS”) that is constructed as a module andprovides the refrigeration system for cooling the cabinet. The CRSmodules can be readily installed and removed from a refrigerated cabinetwhereby such modules are interchangeable and easily serviced.

Typically, the compressor used with a refrigerated cabinet, such as in aCRS installed in the cabinet, is selected to have a capacity that issufficient meet the expected peak cooling load of the refrigeratedcabinet. For example, vending machines must often cool products from anambient temperature to a predetermined storage temperature within apredetermined time period. The initial cooling load generated by loadinga vending machine with ambient temperature products can be relativelysignificant. Oftentimes, the compressor for such vending machines areselected basis of whether the maximum rated capacity of the compressoris sufficient to meet the maximum load that such a vending machine wouldexperience when it is fully loaded with ambient temperature products.When the compressor is selected on this basis, the compressor will oftenbe larger than necessary for the loading conditions most frequentlyexperienced by the vending machine and the efficiency of the compressorwill be less than optimal.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for controllingthe load placed on a compressor by limiting the load placed on one ofthe heat exchangers, e.g., the evaporator of a cooling application, ofthe system to thereby limit the load on the compressor. The presentinvention allows a compressor to be used in applications wherein themaximum anticipated load of the application exceeds the nominal capacityof the compressor. For example, in a refrigeration system used to coolproducts in a refrigerator or vending machine, the peak loads placed onthe evaporator may be limited to avoid exceeding the capacity of thecompressor. As described in greater detail below, this may have only anegligible impact on the time required to cool products placed in such arefrigerated cabinet while significantly improving the efficiency of thesystem.

The invention comprises, in one form thereof, a method of operating avapor compression system wherein the vapor compression system defines aclosed fluid circuit in which a refrigerant is circulated and hasoperably disposed therein, in serial order, a compressor, a highpressure heat exchanger, an expansion device and a low pressure heatexchanger. The method includes operating the system wherein a variablethermal load is placed on a first one of the heat exchangers, monitoringthe thermal load placed on the first heat exchanger and controlling theoperation of the system to limit the thermal load placed on the firstheat exchanger when the thermal load exceeds a first predeterminedvalue.

Oftentimes, and particularly for transcritical cycles, the first heatexchanger will be the low pressure heat exchanger when used in a coolingapplication and the first heat exchanger will be the high pressure heatexchanger when used in a heating application. The thermal load placed onthe first heat exchanger may be monitored in a number of different ways.For example, such monitoring may involve obtaining first and secondvalues indicative of the temperature of the refrigerant at first andsecond locations in the fluid circuit. Or, it may involve obtaining afirst value indicative of the temperature of a heat exchange medium orthe ambient environment and obtaining a second value indicative of anoperating parameter of the vapor compression system. Alternatively, anelectrical motor may be used to drive the compressor and monitoring thethermal load of the first heat exchanger includes monitoring theelectrical current powering the electrical motor.

Controlling the operation of the system to limit the thermal load placedon the first heat exchanger can include controlling the interaction of aheat exchanger medium with the first heat exchanger and may beaccomplished in a number of different ways. For example, the heatexchange medium may be air conveyed by a passageway in communicationwith the first heat exchanger wherein the cross sectional area of thepassageway is controlled. Alternatively, air may be selectivelyrecirculated through a passageway in communication with the first heatexchanger to control the load placed on the first heat exchanger. Or,when the heat exchange medium is air, controlling the interaction of theair with the first heat exchanger may include controlling the operationof an air moving device such as by controlling its operational speed orby controlling the direction at which air is directed by the air movingdevice.

The invention comprises, in another form thereof, a method of operatinga vapor compression system wherein the vapor compression system definesa closed fluid circuit in which a refrigerant is circulated and hasoperably disposed therein, in serial order, a compressor, a highpressure heat exchanger, an expansion device and a low pressure heatexchanger. The method includes coupling the vapor compression systemwith an application wherein a heat exchange medium is communicatedbetween the application and the system, exchanging thermal energybetween the heat exchange medium and a first one of the heat exchangerswherein a variable thermal load is placed on the first heat exchanger bythe heat exchange medium during operation of the system, and controllingthe operation of the system to limit the thermal load placed on thefirst heat exchanger when the thermal load exceeds a predeterminedvalue.

The invention comprises, in yet another form thereof, a vaporcompression system for use with a refrigerant. The system includes aclosed fluid circuit in which the refrigerant is circulated, the fluidcircuit having operably disposed therein, in serial order, a compressor,a high pressure heat exchanger, an expansion device, and a low pressureheat exchanger. The system also includes at least one sensing deviceoperably coupled with the system measuring a value indicative of avariable thermal load placed on a first one of the heat exchangers and aheat exchange subsystem limiting the thermal load placed on the firstheat exchanger when the variable thermal load exceeds a predeterminedvalue.

One aspect of the present invention is that, for a given application, itallows for the use of a compressor having a relatively small capacity.This, in turn, provides several advantages. For example, a smallercapacity compressor is generally less costly than a similar compressorhaving a greater capacity. Limiting the load placed on the vaporcompression system and employing a relatively smaller capacitycompressor will also allow the compressor, heat exchangers and otheraspects of the vapor compression system, e.g., a CRS, to have a smallersize thereby facilitating its use in a greater variety of applications.

Additionally, by limiting the load experienced by the system, the totalcharge of the refrigerant used in the system may be reduced. This may beparticularly advantageous when employing a hydrocarbon refrigerant whichare subject to limitations on the amount of charge that can be used in arefrigeration system.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this inventionwill become more apparent and the invention itself will be betterunderstood by reference to the following description of embodiments ofthe invention taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a sectional view of a vending machine in accordance with thepresent invention;

FIG. 2 is a schematic representation of a vapor compression system;

FIG. 3 is a graph depicting the cooling of two objects having differentBiot numbers;

FIG. 4 is a graph depicting the cooling of a vending machine;

FIG. 5 is a schematic representation of heat exchange subsystem;

FIG. 6 is a schematic representation of alternative heat exchangesubsystem;

FIG. 7 is a schematic representation of another heat exchange subsystem;

FIG. 8 is a schematic representation of the heat exchange subsystem ofFIG. 7; and

FIG. 9 is a schematic representation of a vapor compression systemincluding sensing devices used to monitor a thermal load being placed onthe system.

Corresponding reference characters indicate corresponding partsthroughout the views. Although the drawings represent embodiments of thepresent invention, the drawings are not necessarily to scale and certainfeatures may be exaggerated in order to better illustrate and explainthe present invention. The exemplifications set out herein illustrateembodiments of the invention and such exemplifications are not to beconstrued as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

A vapor compression system 10 in accordance with the present inventionis shown in FIGS. 1 and 2. In the illustrated embodiment, system 10 isused with a refrigerated cabinet 12 that may function as a refrigeratoror vending machine. The illustrated cabinet 12 includes an equipmentcompartment 14 and a refrigerated compartment 16 separated by partitionwall 17. Compartment 14 houses vapor compression system 10 andcompartment 16 is used to store objects being cooled such as beveragecontainers 15 or perishable food products. As best seen in the schematicillustration of FIG. 2, vapor compression system 10 defines a closedfluid circuit in which a refrigerant is circulated and includes, inserial order, a compressor 18, a high pressure heat exchanger 20, anexpansion device 21, and a low pressure heat exchanger 22. Conduits 23provide fluid communication between the various components of system 10.

In general operation, refrigerant vapor enters compressor 18 at arelatively low suction pressure. Compressor 18 compresses and dischargesthe refrigerant vapor at a higher discharge pressure. The compression ofthe refrigerant vapor also increases the temperature of the refrigerantvapor. After being discharged from compressor 18, the high pressurerefrigerant enters high pressure heat exchanger 20. In the illustratedembodiment, the vapor compression system 10 is a convention subcriticalsystem wherein the discharged refrigerant is at a subcritical pressureand high pressure heat exchanger 20 is commonly referred to as acondenser. The present invention, however, may also be used intranscritical systems, such as those using carbon dioxide as arefrigerant, wherein the refrigerant is discharged from the compressorat a supercritical pressure. In such transcritical systems, the highpressure heat exchanger is commonly referred to as a gas cooler insteadof a condenser. Heat exchanger 20 includes an air moving device in theform of fan 24 mounted adjacent to the coils 25 of the heat exchanger20. Fan 24 blows ambient air across the coils of heat exchanger 20 tocool the refrigerant within the coils 25 and thereby condense the highpressure refrigerant into a liquid state. Compartment 14 of cabinet 12is provided with vent openings to allow for the ingress and egress ofthe ambient air being forced across the coils of heat exchanger 20 byfan 24.

After exiting heat exchanger 20, the refrigerant passes throughexpansion device 21 to thereby reduce the pressure of the refrigerant.The reduced pressure refrigerant then enters low pressure heat exchanger22 where it is converted to a gaseous state. Such low pressure heatexchangers are commonly referred to as evaporators. As the refrigerantchanges phase it absorbs thermal energy and cools the air passingthrough the coils 27 of evaporator 22. An air moving device in the formof a fan 28 is mounted adjacent coils 27 of evaporator 22 to move airthrough evaporator 22. The operation of evaporator 22 is discussed ingreater detail below.

Typically, refrigerated cabinets are designed such that the vaporcompression system utilized with the cabinet has a compressor that has amaximum rated capacity that is adequate to meet the anticipated maximumload that the refrigerated cabinet will place on the evaporator. Inrefrigerated cabinets that function as vending machines such as avending machine for dispensing cooled beverage containers, the maximumanticipated load will generally correspond to the load that is createdby entirely filling the cabinet with “warm” product, i.e., beveragecontainers at ambient or room temperature. Typically, vending machinesare required to be capable of cooling all of the beverage containers toa design temperature within a predefined period of time after thevending machine has been fully loaded with warm product.

The cooling load that is generated by such a vending machine will dependupon a number of factors including the warm product temperature, thedesired cooled product temperature, the number of products that thevending machine will hold and also the thermal characteristics of theproduct itself. Different products cool at different rates. The Biotnumber of an object describes the cooling of that object by convection.The Biot number is a dimensionless characteristic that is dependent uponthe heat transfer coefficient governing convective cooling of theobject, the thermal conductivity coefficient and the characteristicdimension of the object. In other words, the Biot number of an object isdependent upon its material and shape. When the Biot number of an objectis small, e.g., considerably less than 1, the cooling of the object willgenerally be limited by the convective boundary conditions and thetemperature gradients within the object will be small. Such a situationwill result when the material has a high thermal conductivity andconvective cooling is relatively weak. When an object has a higher Biotnumber, e.g., greater than 1, the temperature gradient within the objectwill be larger and the internal transfer of thermal energy within theobject may limit the cooling of the object. Such a situation may resultwhen the material of the object has a low conductivity and theconvective cooling of the object is relatively strong.

FIG. 3 illustrates the cooling of two separate objects having differentBiot numbers. The first object is a metal sheet having a Biot number of0.35. As shown in FIG. 3, the metal sheet generates a cooling load thatdecreases at a fairly constant and nearly linear rate over time. Thesecond object is a drink bottle having a Biot number of 6. As shown inFIG. 3, the drink bottle produces a cooling load that decreases at avery rapid rate as it first begins cooling and then begins to cool at amuch slower rate. For example, plastic beverage bottles cool quicklyinitially, releasing a large amount of heat, but then cools more slowly.

FIG. 4 schematically represents the maximum anticipated load for avending machine which has a cooling curve shaped similar to that of thedrink bottle depicted in FIG. 3. FIG. 4 is used to schematically andgraphically represent the concepts discussed herein to clarify suchconcepts but is not necessarily drawn to scale. Generally, the vaporcompression system, also referred to as the refrigeration system, ofsuch a vending machine would be designed to have a peak rated capacitythat was adequate to address anticipated peak load 60. The product willbe at its desired temperature at end point 64 b. A compressor having acapacity rated for peak load 60 will typically have a maximum efficiencyat a load that is above the generally horizontal portion of the coolingload curve. For example, such a compressor may have a maximum efficiencyat a load that corresponds the load represented by dashed line 62. Forsuch a compressor, the compressor will operate at a high efficiency nearline 62, e.g., from approximately point 62 a to point 62 b during thecooling process, but the majority of the cooling required to chill thedrink bottles will take place at a load where the compressor operates ata relatively low efficiency.

By selecting a compressor having a maximum efficiency that correspondsto the load represented by dashed line 64 in FIG. 4, the compressor willbe operated a higher efficiency for a longer period of time during thecooling of a load of warm product. As schematically depicted in FIG. 4,this smaller capacity compressor will be able to operate at grelativelyhigh efficiency from point 64 a to point 64 b. Such a compressor willalso be likely to operate at a more efficient level when maintaining theproduct at the desired product temperature during the time betweenfilling of the vending machine. Such a compressor, however, may not havea capacity that is adequate to address the anticipated peak load 60. Forexample, the maximum rated capacity of such a compressor may correspondto a point 67 above line 66 of FIG. 4.

The present invention enables the use of such a smaller compressor inthe vending machine by limiting the load placed on the vapor compressionsystem when the load exceeds a predetermined value, e.g., line 66 inFIG. 4. By limiting the load placed on the system, the initial coolingof the product may take longer. For example, with reference to FIG. 4,by limiting the maximum load when it exceeds the value of line 66, thetime it takes to cool product from point 60 to point 66 a will likelytake slightly longer than if the load was not so limited. However, afterreaching point 66 a, the time for cooling the product will no longer belimited by the capacity of the compressor. Because the time period frompoint 60 to point 66 a is relatively short in comparison to the totaltime that it takes to cool the product to the desired end point 64 b,the limiting of the maximum load in this initial cool down period willnot have a significant impact on the total time required to cool theproduct while providing significant improvement in the efficiency of thesystem.

Limiting the thermal load placed on vapor compression system 10 andenabling the use of a smaller compressor, can not only improve theefficiency of the system, but may also reduce the cost of the compressorand, potentially, the system as a whole. The limiting of the thermalload may also reduce the total refrigerant charge required by the systemand thereby facilitate the use of hydrocarbon refrigerant which areoften subject to limitations on the total refrigerant charge that may beused in a system. The ability to limit the thermal load of a vendingmachine or similar refrigerated cabinet also provides benefits whenusing CRS modules. Such modules may be removed from a refrigeratedcabinet for servicing or repair and replaced by another CRS module. Byhaving the ability to limit the thermal load placed on such a module,the refrigerated cabinet will be able to accept CRS modules that mightotherwise not have an adequate capacity for the cabinet.

The limiting of the load placed on system 10 will now be discussed withreference to FIGS. 5-8. In the illustrated embodiment, vapor compressionsystem 10 is used to cool a refrigerated cabinet. Air from cabinetinterior 16 is passed through evaporator 22 to cool the air and the airis then returned to cabinet interior 16 where it cools the productslocated therein. Consequently, the load on system 10 is determined bythe thermal load placed on evaporator 22 by the heat exchange medium,i.e., air from cabinet interior 16. By limiting the thermal load placedon evaporator 22, the load on system 10 and compressor 18 can therebyalso be limited. Alternative embodiments of the present invention mayutilize different heat exchanger mediums and/or place the pertinentthermal load on the high pressure heat exchanger. For example, in awater heater application, the thermal load placed on the system may bedetermined by water that is in thermal communication with the highpressure heat exchanger.

FIG. 5 illustrates one embodiment of a heat exchange subsystem 70 thatmay be used with the present invention. In the embodiment of FIG. 5,evaporator 22 and fan 28 are positioned within housing 26. Air fromcabinet interior 16 is drawn into the passageway 29 defined by housing26 through inlet 35 by the action of fan 28. The air is then forcedthrough evaporator 22 where it is cooled and then returns to cabinetinterior 16 through outlet 36. When the thermal load placed onevaporator 22 exceeds a predetermined value, the load may be reduced byrestricting the cross sectional area of inlet 35 of passageway 29. Thecross sectional area of inlet 35 is controlled by an adjustablerestriction member 38. Member 38 may take the form of an electronicallycontrolled baffle member and, in alternative embodiments, instead ofbeing located at the inlet 35 of passageway 29, the restrictor member orother form of baffle for controlling the cross sectional area ofpassageway 29 may be located at an intermediate location in passageway29 either upstream or downstream of evaporator 22 or at the outlet 36 ofpassageway 29.

FIG. 6 illustrates another embodiment 70 a of a heat exchange subsystemthat may be used with the present invention. In this embodiment, housing26 a defines a bypass channel 40 having an inlet 41 located downstreamof evaporator 22 and an outlet 42 located upstream of evaporator 22. Arestrictor member 38 a which may also be an electronically controlledbaffle member controls the air flow into inlet 41 of bypass channel 40.To limit the thermal load placed on evaporator 22, restrictor member 38a is moved to a position where inlet 41 is open and air enters bypasschannel 40 after passing through evaporator 22. The air in bypasschannel 40 is then returned to passageway 29 upstream of evaporator 22where it acts to reduce the average temperature of the air streamflowing across evaporator 22 and thereby reduce the thermal load beingplaced on evaporator 22.

FIGS. 7 and 8 illustrate another embodiment 70 b of a heat exchangesubsystem that may be employed with the present invention. In thisembodiment, housing 26 b defines a recirculation channel 46. Channel 46has an inlet 47 in fluid communication with passageway 29 at a locationbetween inlet 35 and evaporator 22 and an outlet 48 that is in fluidcommunication with passageway 29 between inlet 35 and inlet 47. Fan 28 ais repositionable, and to limit the thermal load placed on evaporator22, the position of fan 28 a is moved from that depicted in FIG. 7 tothe position shown in FIG. 8. Motor assembly 19 a includes anelectronically controlled servo motor to move the position of fan 28 a,however, other means of moving the position of fan 28 a may also beemployed. Arrows 49 represent the direction in which air is moved by fan28 a in these different positions. When fan 28 a is in the positionshown in FIG. 7, fan 28 a moves air in a first direction substantiallyperpendicular to the lengthwise direction of the coils of evaporator 22and which maximizes the flow of air through evaporator 22. When fan 28 ais moved into the position shown in FIG. 8, fan 28 a moves air in asecond direction that is directed towards inlet 47 of recirculationchannel 46. When fan 28 a is in this second position, a greater quantityof air is directed through channel 46 and the air flow throughevaporator 22 is reduced thereby reducing the thermal load placed onevaporator 22.

The air moving device 28, 28 a used with the embodiments shown in FIGS.5-8 may be a variable speed device wherein varying the operating speedof the device varies the flow rate of the air moved by the device. Forexample, using variable speed fans 28, 28 a allows the fan blade speedto be varied to reduce the mass flow rate of air through evaporator 22.The adjustment of the operating speed of fan 28, 28a, by itself, in theembodiments of FIGS. 5-8 is capable of altering the thermal load beingplaced on evaporator 22. For example, if the embodiment depicted in FIG.5 did not include restrictor 38, the operating speed of fan 28 could bereduced to thereby limit the thermal load placed on evaporator 22.Alternatively, by combining a variable speed fan with the embodiments ofFIGS. 5-8, the operating speed of the fan can be controlled (e.g.,reducing the operating speed) to supplement the thermal load limitingeffects of restrictor 38, bypass channel 40 and recirculation channel 46described above.

Monitoring the thermal load being placed on one of the high or lowpressure heat exchangers of the system to determine when it has exceededa predefined value and should be limited can be accomplished in a numberof different ways. In the illustrated embodiments, it is the thermalload on evaporator 22 that is monitored and system 10 includes acontroller 30 which is in communication with one or more sensing devicesto enable controller 30 to monitor the load. FIG. 9 schematicallydepicts system 10, including a heat exchange subsystem 70, controller 30and various sensing devices 32 a-32 j, not all of would be used in asingle system.

One of the primary objectives of limiting the thermal load being placedon evaporator 22 is to prevent the overloading of compressor 18.Consequently, an effective way of indirectly monitoring the load placedon evaporator 22 is to monitor the electrical current required to powercompressor 18 using a sensing device 32 a in communication withcontroller 30. When the current supplied to compressor 18 exceeds apredetermined value, the heat exchange subsystem is controlled to reducethe load being placed on evaporator 22. For example line 31 extendingfrom controller 30 could be in communication with restrictor 38 tocontrol the position of restrictor 38, with fan 28 to control theoperating speed of the fan, or with another device, such as thosediscussed in greater detail above, capable of limiting the thermal loadbeing placed on evaporator 22. Controller 30 may also be programmed sothat it varies its response as the current supplied to compressor 18varies. For example, the controller 30 could be programmed to moverestrictor 38 to vary the cross sectional area of inlet 35 wherein theopen area of inlet 35 is progressively diminished as the currentsupplied to compressor 18 progressively increases beyond a predefinedvalue. In other embodiments, the controller may also be programmed toreduce the operational speed of fan 28 when the current to compressor 18exceeds a predefined value. The reduction of the operating speed of fan28 may be to a single predefined lower operating speed or be a stepwisereduction which progressively lowers the speed of fan 28 as the currentto compressor 18 progressively increases beyond the predefined value.

Even when employing a fan or other air moving device that has only asingle operating speed, controller 30 may be programmed to deactivatethe air moving device when the current supplied to compressor 18. Forexample, restrictor 38 may be employed to progressively restrict inlet35 after the current supplied to compressor 18 reaches and then exceedsa first predefined value and, if the current supplied to compressor 18reaches a second, higher predefined value, controller 30 could controlfan 28 by deactivating it.

With reference to FIG. 4, a first predefined value of the currentsupplied to compressor 18 could correspond to line 66. When this valueis exceeded, the thermal load being placed on evaporator 22 would beginto be limited. The current value selected to define line 66 may beadvantageously selected below the maximum current level of compressor18, i.e., a value corresponding to point 67 in FIG. 4, to allow for thethermal load to continue to increase slightly after beginning to limitthe thermal load. In other words, although the thermal load is beinglimited, it is possible that the rate of increase in the thermal loadexceeds the rate of limitation on the thermal load. The controller maybe programmed to shut the system down if the limiting of the thermalload fails to prevent the thermal load from exceeding point 67.

Other sensors depicted in FIG. 9 that may be used to monitor a thermalload being placed on system 10 either directly or indirectly includesensors 32 a-32 j which may be used in various combinations to monitorthe thermal load being placed on system 10. Persons having ordinaryskill in the art are familiar with the use of sensing devices to monitorthe operation of a vapor compression system and the thermal load beingplaced thereon and sensing devices 32 b-32 j schematically depicted inFIG. 9 may be used in various combinations to monitor the thermal loadbeing placed on system 10 as is known in the art.

For example, sensors 32 b and 32 c may be used together to monitor thethermal load placed on heat exchanger 22 or may be used in combinationwith additional sensing devices. Sensor 32 b is used to measure the airtemperature within compartment 16 of cabinet 12, alternatively, sensor32 b could be located in housing 32 shown in FIG. 1 and measure theexternal air temperature. Sensor 32 c measures the temperature of theheat exchange medium, i.e., air, after it has been cooled by evaporator22.

Sensors 32 e and 32 f which respectively measure the temperature and/orpressure of the refrigerant at the inlet and outlet of compressor 18 mayalso be used to indirectly monitor the thermal load being placed onevaporator 22. Alternatively, sensors 32 g and 32 h which respectivelymeasure the temperature of the refrigerant at the inlet and outlet ofthe high pressure heat exchanger 20 can be used to indirectly monitorthe thermal load on heat exchanger 22. Sensors 32 i and 32 j whichrespectively measure the temperature of the refrigerant at the inlet andoutlet of the low pressure heat exchanger 22 may also be used to monitorthe load on heat exchanger 22. As is known in the art, it is alsopossible to measure various other system operating parameters, e.g., thepressure of the refrigerant within the high pressure heat exchanger in atranscritical system, when determining the load being placed on one ofthe heat exchangers in the system.

For example, when employing sensors 32 c and 32 d to measure thetemperature of the air after it has passed through evaporator 22 and thetemperature of the refrigerant within evaporator 22 the differentialbetween the two temperatures may be used to provide a value indicativeof the thermal load being placed on the evaporator. Sensors 32 c and 32d may be employed with heat exchange subsystem 70 a wherein air isrecirculated through bypass channel 40 to reduce the thermal load placedon evaporator 22 when the temperature differential measured by sensors32 c and 32 d exceeds a predetermined value.

While this invention has been described as having an exemplary design,the present invention may be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains.

1. A method of operating a vapor compression system, the vaporcompression system defining a closed fluid circuit in which arefrigerant is circulated and having operably disposed therein, inserial order, a compressor, a high pressure heat exchanger, an expansiondevice and a low pressure heat exchanger, said method comprising:applying a variable thermal load on a first one of the heat exchangers;monitoring the thermal load placed on the first heat exchanger; andcontrolling the operation of the system to limit the thermal load placedon the first heat exchanger when the thermal load exceeds apredetermined value.
 2. The method of claim 1 wherein monitoring thethermal load of the first heat exchanger comprises obtaining a firstvalue indicative of the temperature of the refrigerant at a firstlocation in the fluid circuit and obtaining a second value indicative ofthe temperature of the refrigerant at a second location in the fluidcircuit.
 3. The method of claim 2 wherein the first location isproximate an inlet to the low pressure heat exchanger and the secondlocation is proximate an outlet of the low pressure heat exchanger. 4.The method of claim 2 wherein the first location is proximate an inletto the high pressure heat exchanger and the second location is proximatean outlet of the high pressure heat exchanger.
 5. The method of claim 2wherein the first location is proximate an inlet to the compressor andthe second location is proximate an outlet of the compressor.
 6. Themethod of claim 1 wherein monitoring the thermal load of the first heatexchanger comprises obtaining a first value indicative of thetemperature of a heat exchange medium in thermal communication with thefirst heat exchanger and obtaining a second value indicative of anoperating parameter of the vapor compression system.
 7. The method ofclaim 6 wherein said first heat exchanger is the low pressure heatexchanger and said second temperature is the discharge temperature ofair cooled by the first heat exchanger.
 8. The method of claim 1 whereinan electrical motor drives the compressor and monitoring the thermalload of the first heat exchanger comprises monitoring the electricalcurrent powering the electrical motor.
 9. The method of claim 1 whereincontrolling the operation of the system comprises controlling theinteraction of a heat exchange medium with the first heat exchanger. 10.The method of claim 9 wherein the heat exchange medium is air conveyedby a passageway in communication with the first heat exchanger andcontrolling the interaction of the air with the first heat exchangercomprises controlling the cross sectional area of the passageway. 11.The method of claim 9 wherein the heat exchange medium is air conveyedby a passageway in communication with the first heat exchanger andcontrolling the interaction of the air with the first heat exchangercomprises selectively recirculating air in the passageway.
 12. Themethod of claim 9 wherein the heat exchange medium is air andcontrolling the interaction of the air with the first heat exchangercomprises controlling the operation of an air moving device incommunication with the first heat exchanger.
 13. The method of claim 12wherein controlling the operation of the air moving device comprisescontrolling the operating speed of the air moving device.
 14. (canceled)15. A method of operating a vapor compression system, the vaporcompression system defining a closed fluid circuit in which arefrigerant is circulated and having operably disposed therein, inserial order, a compressor, a high pressure heat exchanger, an expansiondevice and a low pressure heat exchanger, said method comprising:coupling the vapor compression system with an application wherein a heatexchange medium is communicated between the application and the system;exchanging thermal energy between the heat exchange medium and a firstone of the heat exchangers, wherein a variable thermal load is placed onthe first heat exchanger by the heat exchange medium during operation ofthe system; controlling the operation of the system to limit the thermalload placed on the first heat exchanger when the thermal load exceeds apredetermined value.
 16. The method of claim 15 wherein the applicationis a refrigerated cabinet, the first heat exchanger is the low pressureheat exchanger and the heat exchange medium is air that is cooled by thefirst heat exchanger.
 17. The method of claim 16 wherein controlling theoperation of the system comprises controlling the passage of air overthe first heat exchanger.
 18. The method of claim 17 wherein controllingthe passage of air over the first heat exchanger comprises controllingthe area of a passageway through which flows the air passing over thefirst heat exchanger.
 19. The method of claim 17 wherein controlling thepassage of air over the first heat exchanger comprises selectivelyrecirculating air within a passage in communication with the first heatexchanger.
 20. The method of claim 17 wherein controlling the passage ofair over the first heat exchanger comprises controlling the operation ofan air moving device forcing the passage of air over the first heatexchanger.
 21. The method of claim 20 wherein controlling the operationof the air moving device comprises controlling the operating speed ofthe air moving device.
 22. (canceled)
 23. The method of claim 15 whereinan electrical motor drives the compressor and monitoring the thermalload of the first heat exchanger comprises monitoring the electricalcurrent powering the electrical motor.
 24. The method of claim 15wherein monitoring the thermal load of the first heat exchangercomprises obtaining a first value indicative of the temperature of theambient environment and obtaining a second value indicative of anoperating parameter of the vapor compression system.
 25. The method ofclaim 15 wherein monitoring the thermal load of the first heat exchangercomprises obtaining a first value indicative of the temperature of therefrigerant at a first location in the fluid circuit and obtaining asecond value indicative of the temperature of the refrigerant at asecond location in the fluid circuit.
 26. A vapor compression system foruse with a refrigerant, said system comprising: a closed fluid circuitin which the refrigerant is circulated, the fluid circuit havingoperably disposed therein, in serial order, a compressor, a highpressure heat exchanger, an expansion device, and a low pressure heatexchanger; at least one sensing device operably coupled with said systemmeasuring a value indicative of a variable thermal load placed on afirst one of said heat exchangers; and a heat exchange subsystemlimiting the thermal load placed on the first heat exchanger when thevariable thermal load exceeds a predetermined value.
 27. The vaporcompression system of claim 26 wherein said at least one sensing devicecomprises a first temperature sensor positioned at a first location insaid fluid circuit and a second temperature sensor positioned at asecond location in said fluid circuit.
 28. The vapor compression systemof claim 26 wherein said at least one sensing device comprises a firsttemperature sensor positioned to measure an ambient temperature and asecond temperature sensor positioned to measure a temperature indicativeof an operating parameter of the vapor compression system.
 29. The vaporcompression system of claim 26 further comprising an electrical motorcoupled to said compressor and driving said compressor and wherein saidat least one sensing device senses the electrical current powering saidelectrical motor.
 30. The vapor compression system of claim 26 whereinsaid system is a modular assembly removably couplable to an application.31. The vapor compression system of claim 26 further comprising acabinet and an air passage providing communication between the firstheat exchanger and an interior volume of the cabinet, the first heatexchanger being the low pressure heat exchanger.
 32. The vaporcompression system of claim 31 wherein said heat exchange subsystemcontrols the flow of air through said air passage.
 33. The vaporcompression system of claim 32 wherein heat exchange subsystem comprisesan adjustable restriction member, adjustment of said restriction membervarying the cross sectional area of said air passage.
 34. The vaporcompression system of claim 32 wherein said heat exchange subsystemfurther comprises a second passage in communication with said airpassage at first and second locations wherein air is recirculatablethrough said air passage through said second passage.
 35. (canceled) 36.The vapor compression system of claim 31 wherein said heat exchangesubsystem comprises an air moving device forcing the passage of air oversaid first heat exchanger and wherein the variable operation of said airmoving device controls the flow of air through said air passage.
 37. Thevapor compression system of claim 36 wherein said air moving device hasa variable operating speed and varying said operating speed varies theflow rate of air through said air passage.
 38. (canceled)